1.
The invention relates to a process for the production of alkali metal silicates from crystalline SiO.sub.2 -containing material and aqueous alkali metal hydroxide solution at elevated temperature and under normal pressure.
2. Statement of Related Art
Alkali metal silicates, e.g. water glasses, are produced in large quantities and are used both in solution and also as solids in many fields. Among such are washing and cleaning agents, adhesives, paints, the floatation of ore, and water treatment. They also serve as raw materials for the production of zeolites as well as silicic acids, silica sols, and silica gels (Buchner et. al., Industrielle Anorganishce Chemie (Industrial Inorganic Chemistry), Verlag Chemie, 1984 p. 333).
Waterglass solutions are usually characterized by two physical values. Firstly, by the molar ratio SiO.sub.2 /M.sub.2 O, hereafter referred to as the modulus, and by the solids content, i.e. the proportion by weight of SiO.sub.2 and M.sub.2 O in the solution, where M stands for Na or K. Both factors influence the viscosity of the alkali metal silicate solution.
The maximum solubility of an alkali metal silicate with a specific modulus can be determined from tables and diagrams. Generally, at higher alkali contents, i.e. a lower modulus, higher solids contents can be achieved in the solution. Waterglasses up to a modulus of 4.3 are obtained via melting processes. Such melting processes have already been known since the last century; the only process still used nowadays is the conversion of quartz sand with soda at temperatures around 1500.degree. C. (Winnacker-Kuchler, Chemische Technologie (Chemical Technology), C. Hanser Verlag, 4th Edition (1983), Volume 3, Anorganishce Technologie II (Inorganic Technology II), page 58 et seq.). Only a small proportion of the alkali metal silicate prepared in this way is sold as solid glass. Most of it is subsequently dissolved in water. For glasses with a modulus &gt;2.0 the reaction velocity at the reflux temperature of the solution is not satisfactory, so that a pressurized decomposition at 4-6 bar and 150.degree. C. is preferred.
Low molecular ratio waterglasses (modulus &lt;2.7) which are richer in alkali can also be prepared hydrothermally e from quartz sand and concentrated aqueous sodium hydroxide solution. For that purpose, because of the low reactivity of quartz sand, raised temperatures and increased pressure are necessary. In industry two processes are used: nickelplated rotating pressure dissolvers at temperatures of 200 to 220 .degree. C. and tubular reactors at temperatures of 250.degree. to 260 .degree. C. (Winnacker-Kuchler, loc. cit., page 61 et seq.).
Finely divided, amorphous silicic acids dissolve ex othermically in alkali lyes. Thus, occasionally particularly pure alkali metal silicate solutions are obtained by the reaction of pyrogenic or precipitated silicic acid with an alkali lye. Apart from special cases, this process is & too costly.
Amorphous silicic acid also occurs as by-product or waste in various industrial processes. Numerous processes for employing such silicic acids are documented.
According to Przem. Chem. 67(8) (1988) 384-6 (Chemical Abstracts 109:214144f), sodium silicate solutions with a middling modulus can be obtained from the waste silicic acid from the preparation of AlF.sub.3 and HF.
According to JP-76 17519 (Chemical Abstracts, 86:19 116 g) dust from the preparation of ferrosilicon contains about 90 wt-% of highly-reactive SiO.sub.2, which can be converted with 8.1% sodium hydroxide solution at temperatures around 90.degree. C. to a waterglass solution with a high modulus.
As another alkali metal silicate, solid crystalline anhydrous sodium polysilicate (Na.sub.s SiO.sub.2).sub.00 has industrial importance as builder component in washing and cleaning agents. This sodium silicate with the empirical composition Na.sub.2 SiO.sub.3 contains endless chains of SiO.sub.4 tetrahedrons, which are bonded to the sodium atoms via bridges. Such chain silicates are called "inosilicates" in mineralogy and "polysilicates" in chemistry. The term "metasilicate" is widely used but incorrect. Hereinafter only the term "sodium polysilicate" is employed.
Anhydrous sodium polysilicate is prepared on a major industrial scale by a tempering reaction of quartz sand and soda in a rotary tube furnace at about 950.degree. C. (Buchner et. al. Industrielle Anorganische Chemie, Verlag Chemie, 1984, p. 333). For this purpose the reaction time is about 45 min. At still higher temperature, when reaction times are, however, shorter, the polysilicate can also be obtained by melting sand and soda (Ullmann's Encylkopadie der technischen Chemie (Ullmann's Encyclopedia of Technical Chemistry), Verlag Chemie, 1982, Vol. 21, p. 412).
Furthermore, U.S. Pat. No. 3,532,458 describes the hydrothermal production of sodium polysilicate, starting from quartz sand. Temperatures of about 200.degree. C. under increased pressure are necessary for complete reaction of the quartz sand with aqueous sodium hydroxide solution.
DE-AS 15 67 572 proposes the preparation of anhydrous, crystalline alkali metal silicates, preferably sodium polysilicate, by producing a film on finely divided, solid alkali metal silicate, which is heated to a temperature above 130.degree. C. and kept in continual motion, by spraying an aqueous alkali metal silicate solution onto it and evaporating off the water by means of an additional hot stream of gas, the coating and drying steps being repeated until the size of the crystalline anhydrous alkali metal silicate particles has increased to the desired extent. Generally, a part of the substantially anhydrous alkali metal silicate produced is recycled to the continuous process as a starting component.
The disadvantages of this last-mentioned process are that in the sodium polysilicate which is kept constantly in movement one needs a large number of sodium polysilicate particles as the inoculation basis and that therefore a very large proportion of the spray granulate produced must be re-ground and recycled, so that ultimately the yield of this process is low.
It is further known from DE-PS 968 034 to produce solid sodium polysilicate containing water of crystallization in such a way that finely-divided silicic acid, such as quartz sand or quartz flour, and aqueous sodium hydroxide e solution are homogeneously mixed together in a ratio which approximately corresponds to the ratio of alkali metal oxide: SiO.sub.2 in the product to be prepared, the mixture is continuously conveyed into a reaction tube against the pressure prevailing therein and passed through the reaction tube at temperatures above approximately 175.degree. C. and under increased pressure, with the proviso that an even distribution of the silicic acid in the mixture is ensured by regulating the linear flow rate. Subsequently the hot reaction product is allowed to leave the reaction tube through an expansion valve, so that the initially higher water content of the reaction product formed is reduced, as a result of the water evaporation occurring upon expansion, to the desired water content of the end product. In this way it is possible to produce hydrated sodium polysilicate with less than 9 moles of water of crystallization.
It is further known from NL-OS 78 02 697 to produce sodium silicate solutions by passing sand with sodium hydroxide solution under raised pressure and at a temperature of at least 200.degree. C. through a tube, which can be used for the continuous treatment of bauxite and which is known from e.g. DE-OS 21 06 198 as well as DE-OS 25 14 339. For the production of metasilicate products one works preferably at a temperature of 200.degree. to 240 .degree. C.; for the production of products with a higher ratio of SiO.sub.2 :Na.sub.2 O one preferably uses temperatures from 240.degree. to 280 .degree. C. The pressure in the tube preferably lies in the range between 10,000 and 20,000 kPa. However, according to the processes described in this Offenlegungsschrift (published German patent application) only solutions and not solid products are produced.
DE-OS 31 24 893 describes a process for the production of anhydrous sodium polysilicate by treatment of quartz sand and/or quartz flour with concentrated aqueous sodium hydroxide solution under pressure at a temperature in the range of 200.degree. to 400 .degree. C.
RO 75620 (Chemical Abstracts 100:24023U) describes a process for the production of crystalline sodium polysilicate with a modulus of 1:1 from silicon dioxide-containing waste products from the manufacture of fertilizers. This process is thereby characterized, that the solution containing sodium polysilicate must first be filtered, in order to remove impurities, before the filtrate is concentrated. Then crystallization occurs on cooling the solution down to a temperature of 10.degree. to 15 .degree. C.
SU-434060 (Chemical abstracts 82:45938w) describes a process for the production of sodium polysilicate from volcanic ash.
JP-73/16438 (Chemical Abstracts 80:17050R) describes a process for the production of sodium polysilicate-containing solutions from flue gas residues.
These three last-mentioned processes however have the disadvantage that the SiO.sub.2 sources used are contaminated and the removal of the impurities makes for considerable difficulties, so that up until now these processes have not proved successful (Winnacker-Kuchler, loc. cit., page 61).